In vehicular cement mixers the mixing drum is rotated by a hydraulic drive generally energized by the vehicle's engine. The drive includes a hydrostatic motor connected via a torque-increasing power transmission to the drum, the cooling of the motor being effected by an oil bath. The oil is in turn cooled by means of a heat exchanger and an air blower generally driven by a separate power source such as an electric motor.
The oil from the motor-cooling bath may be circulated through the heat exchanger either by a pump or by a thermal siphoning effect. To increase the cooling performance the oil from the bath is partitioned into several streams which are channeled through respective thin-walled tubes or pockets of the heat exchanger. The air stream from the blower flows around the tubes and baffles of the heat exchanger and may be directed to subsequently blow on an outer surface of the drive housing.
Upon actual application of the above-described cooling system for relatively long periods of time, various difficulties arise. Dust and dirt frequently present in the air at construction sites form a coating on the heat exchangers which decreases the efficiency, i.e. the rate, of heat transfer and thereby creates oil feed and drive problems. Attempts to clean the heat exchangers frequently result in damage to the thin-walled circulation tubes.
Other disadvantages of cement mixers having such cooling systems include the relative inaccessibility of the cooling surfaces to inspection and maintenance and the impossibility of improving the cooling capacity without greatly raising energy expenditure. Generally the lubrication of the mechanical drive is separate from the cooling of the hydrostatic drive, different kinds of oil being required for the two functions. The separation of mechanical and hydrostatic oil-circulating systems more than doubles the difficulty of budgeting the oil supply.